This project has two parts, one investigating the communication between parasympathetic nerve development and SMG epithelial morphogenesis, and the second identifying embryonic mouse salivary gland stem/progenitor cell populations within the gland. The maintenance of progenitors as a reservoir of undifferentiated cells is required for organ development and regeneration. Parasympathetic nerves are a vital component of the progenitor niche during development. Injured adult organs do not regenerate after parasympathectomy, and there are few treatments to improve organ regeneration or prevent damage, particularly that caused by therapeutic irradiation. Restoring parasympathetic function with the neurotrophic factor neurturin increases epithelial organ regeneration after irradiation. We used SMG explant culture and injured the tissue with irradiation. The progenitors survived, parasympathetic function was diminished, and epithelial apoptosis reduced expression of neurturin, which consequently increased neuronal apoptosis. Treatment with neurturin reduced neuronal apoptosis, restored parasympathetic function, and increased epithelial regeneration. Furthermore biopsies of adult human salivary glands damaged by irradiation also had reduced parasympathetic innervation. We propose that neurturin will protect the parasympathetic nerves from damage and improve regeneration. We are using viral vectors that express neurotrophic factors to infect salivary glands in vivo and ex vivo to improve the function of surviving nerves in repairing the gland. We are also studying the cell surface receptor Kit, which has been used to isolate stem/progenitor cells from adult SMGs. These Kit+ progenitor/stem cells can be transplanted into irradiated adult SMGs to regenerate the damaged tissue and restore secretory function. Both the formation and regeneration of SMGs require epithelial progenitor expansion to engineer, maintain and repair the branched tissue architecture. Identifying the mechanisms that control progenitor expansion will inform therapeutic organ regeneration. We discovered that combined KIT and FGFR2b signaling specifically increase distal progenitor expansion during salivary gland organogenesis. FGFR2b signaling upregulates the epithelial KIT pathway so that combined KIT/FGFR2b signaling, via separate AKT and MAPK pathways, amplifies FGFR2b-dependent transcription. Combined KIT/FGFR2b signaling selectively expands the number of KIT+K14+SOX10+ distal progenitors. We analyzed SMGs from mice with mutations in the Kit receptor. The SMGs are smaller and have reduced number of Kit+ distal progenitor cells. However, the genetic loss of Kit signaling unexpectedly depletes the K5+ proximal progenitors. This occurs because the distal progenitors produce neurotrophic factors, such as neurturin, that support gland innervation and maintain the proximal progenitors. Furthermore, a rare population of KIT+FGFR2b+ cells is present in adult glands, in which KIT signaling also regulates epithelial-neuronal communication during homeostasis. Our findings provide a framework to direct regeneration of branched epithelial organs.